The invention relates to new and useful improvements in apparatus and method for the removal of zinc and zinc alloy coatings from steel scrap and recovery of zinc. More particularly, the invention relates to moving galvanized scrap sequentially through a plurality of leaching tanks connected in series, transferring a caustic leaching solution containing an oxidant through the leaching tanks in a direction opposite to the movement of the scrap, transferring exhausted leaching solution when the oxidant is substantially depleted to means for removing zinc to recycle the solution and then returning the solution back through the tanks after zinc is removed.
A major concern to the steel industry is the growing quantity of basic oxygen furnace (BOF) and electric arc furnace (EAF) baghouse and wet scrubber dusts containing elevated levels of heavy metals such as zinc, cadmium and lead. In previous years when galvanized steels were produced in smaller volumes, these dusts could be recycled through the normal sinter plantblast furnace-BOF processing loop. Due to accelerating trends towards greater use of galvanized flat rolled steel in autobodies for corrosion protection, increasing amounts of galvanized scrap at automobile stamping plants have been accumulating. Recycling of large amounts of galvanized steel scrap directly through a BOF or an EAF is undesirable because zinc vaporizes during the melting process and significantly increases the zinc content of the flue dust. Flue dust having a high zinc content, i.e., zinc oxide, is unacceptable feedstock for recycling as sinter to a blast furnace because zinc escapes from the sinter by being vaporized after it is reduced to metallic zinc in the blast furnace. The zinc vapors condense on furnace refractories and can destroy the refractory lining. Consequently, much of the flue dust from a BOF containing zinc is disposed of as a waste because the available recycling processes are uneconomical. This represents poor utilization of resources since the iron and zinc units are disposed of rather than recovered.
Coating metals have heretofore been removed from steel scrap both for recovery of the coating metal and recycling of the deplated steel scrap to a steel making furnace. U.S. Pat. No. 3,905,882 discloses a process for counterflow sulfuric acid leaching of galvanized steel scrap and recovery of metallic zinc from the solution in a separate electrolytic cell. Fresh galvanized scrap is placed into a first tank containing concentrated acid. After this scrap is about one-half dezinced, fresh scrap is placed into a second tank. The now weak acid from the first tank is circulated to the second tank and concentrated acid returned from the electrolytic cell is recirculated to the first tank. Although acids are good deplaters of zinc coatings, acids generally are unacceptable because acids severely corrode iron based materials and require higher cost polymeric based materials for the dezincing plant equipment and acids dissolve iron from the scrap during the leaching process thereby requiring expensive purification schemes to produce a high purity metallic zinc by-product.
Canadian patent 1,193,996 discloses a process using an electrolytic cell both for dezincing galvanized steel scrap and for recovering metallic zinc from the leaching solution. This process uses an electrolytic cell containing mercury and an alkaline solution. Scrap is carried into the leaching solution on an anode conveyor chain with the mercury acting as the cathode for deposition of zinc. The formed mercury amalgam is recirculated through another cell wherein the zinc is separated onto another cathode.
U.S. Pat. No. 4,056,450 discloses a tank for detinning steel scrap and recovery of tin. The tank includes a rotatable perforated anode drum and a caustic leaching solution. Shredded scrap is fed to the drum through a chute, a helical member moves the scrap through the drum and detinned scrap exits the discharge end of the drum onto a conveyor. Tin is electrowinned from the leaching solution onto cathodes suspended from a bus bar within the tank. Leaching solutions having concentrations of 1-20% NaOH or 1-40% KOH are disclosed with the operating temperature maintained at 82.degree. C.
The processes disclosed in either of the latter two patents are unacceptable because they are very inefficient. Using electric current to anodically remove a coating metal from steel scrap by dissolving in a leaching solution and simultaneously electrowinning the dissolved metal by cathodic deposition from a leaching solution within a leaching tank results in low utilization of the current passing through the leaching tank. Anodic dissolution of coating metal from steel scrap requires "line-of-sight" alignment between the coated steel surface (anode) and the cathode for high electrical efficiency. Dissolution may not be effective during high volume scrap processing or when processing scrap compressed into bales.
Because of increasing usage of zinc coated steels, uncoated steel scrap will become increasingly less available and its cost will escalate. Usage of greater amounts of zinc coated steel scrap in BOF or EAF furnaces will result in increased concentrations of heavy metals in dust captured in furnace gas cleaning equipment. Accordingly, there remains a need for a process for zinc removal from galvanized steel scrap that is fast and relatively inexpensive. Furthermore, there is a need for a process that is environmentally safe, does not produce waste by-products which are expensive or difficult to handle, does not cause operator discomfort and does not corrode the operating equipment.